TLRs and their ligands

Cells of the innate immune system, such as macrophages and monocytes possess receptors for “common features of foreign”. These receptors belong to the toll-like receptor (TLR) family. To date, ten TLR receptor family members are known, each member binding a different molecule-type that is characteristic for microbial antigens. The respective ligands for the individual TLR members are specified above each TLR in the figure. If a TLR binds its ligand, that results in activation of the macrophage/monocyte, inducing the cell to secrete cytokines such as the ‘endogenous pyrogen”, IL-6. Through this mechanism, by activating “endogenous pyrogen”/IL6 production microbial products induce the fever reaction, that is, become “exogenous pyrogens”.